coverage: Simplify sorting of coverage spans extracted from MIR

Switching to `Ordering::then_with` makes control-flow less complicated, and
there is no need to use `partial_cmp` here.

compiler/rustc_mir_transform/src/coverage/graph.rs

@@ -199,12 +199,8 @@ impl CoverageGraph {
     }
 
     #[inline(always)]
-    pub fn rank_partial_cmp(
-        &self,
-        a: BasicCoverageBlock,
-        b: BasicCoverageBlock,
-    ) -> Option<Ordering> {
-        self.dominators.as_ref().unwrap().rank_partial_cmp(a, b)
+    pub fn cmp_in_dominator_order(&self, a: BasicCoverageBlock, b: BasicCoverageBlock) -> Ordering {
+        self.dominators.as_ref().unwrap().cmp_in_dominator_order(a, b)
     }
 }